The importance of zooplankton‐protozoan trophic couplings in Lake Michigan

Abstract

The importance of the zooplankton‐protozoan trophic coupling was determined experimentally by measured changes in protozoan growth rates with increasing zooplankton biomass. In five of six experiments conducted in Lake Michigan, a significant inverse relationship between protozoan growth and zooplankton biomass was observed (avg r2 = 70%). Zooplankton clearance rates on protozoan assemblages [range, 1.0–6.2 ml (µg dry wt)‒1 d‒1] were comparable to those previously measured for phytoplankton which suggested that protozoa are important prey for zooplankton. Clearance rates on individual protozoan taxa [0–15.6 ml (µg dry wt)‒1 d‒1] were size‐dependent. Rates were greatest for taxa <20 µm in size (mainly nanoflagellates and small ciliates). In contrast to findings for phytoplankton, no evidence emerged for grazer resistance nor growth enhancement by planktonic protozoa in response to grazing. The high flux rates for macrozooplankton on heterotrophic nanoflagellates observed in all experiments (0.2–6.0 µg C liter‒1 d‒1) provided evidence that a large fraction of picoplankton C may be directly transferred to higher trophic levels via a picoplankton‐flagellate‐zooplankton coupling.